1. Field of the Invention
The present invention relates to a sheet conveying apparatus and an image forming apparatus, and more particularly to a sheet conveying apparatus that corrects a skew feeding of a sheet and a position of a sheet in a width direction, and an image forming apparatus provided with the sheet conveying apparatus.
2. Description of the Related Art
In an image forming apparatus such as a copying machine, printer, or facsimile machine, when a sheet that is to be conveyed is skewed, or a deviation is caused at the position (hereinafter referred to as a “lateral registration position”) in the width direction orthogonal to a sheet conveying direction, an image is unfavorably formed on a sheet with an image position deviated. In view of this, the sheet conveying apparatus of the image forming apparatus is provided with a skew feeding correction portion that aligns a posture or position of a sheet before a sheet is conveyed to an image forming portion.
The skew feeding correction portion is generally provided at the upstream side of a transfer portion for transferring an image onto a sheet. For example, the skew feeding correction portion performs a skew feeding correction of a sheet on a side registration basis in which a positional deviation of a sheet is corrected based on a side end of the sheet that is currently conveyed (see U.S. Pat. No. 6,273,418).
The skew feeding correction portion that corrects the skew feeding of the sheet on the side registration basis includes an abutment reference member provided along a sheet conveying direction at one side of a sheet conveying path, and plural pairs of skew-feed rollers (skewed rollers) arranged on the sheet conveying path. The abutment reference member has a reference surface that is substantially parallel to the sheet conveying direction. The plural pairs of skew-feed rollers are arranged along the reference surface in the sheet conveying direction. The skew of the sheet with respect to the sheet conveying direction is corrected in such a manner that the sheet, which is currently conveyed, is conveyed as being skewed toward the reference member by the pair of skew-feed rollers, and the side end of the sheet abuts on the reference surface so as to allow the sheet to be along the reference surface. The position of the side end of the sheet in a direction orthogonal to the sheet conveying direction can be specified by the reference surface, whereby the positional deviation of the sheet in the width direction can be corrected based on the position of the reference surface.
However, as illustrated in FIG. 18A, when the sheet S is nipped by a first pair of skew-feed rollers 32a located at the most upstream side, force Fp that is exerted in a direction reverse to the sheet conveying direction is applied to a center of gravity of the sheet S. When the distance from the center of gravity to the side end of the sheet is defined as Lp, a moment M (=Fp×Lp) that the sheet tries to turn in a direction of an arrow in FIG. 18A is caused on the side end of the sheet S. The sheet turns due to the moment M, and the side end of the sheet S abuts on the abutment reference member 31, whereby the skew feeding is corrected. Simultaneously, a deflection is generated on the side end of the sheet S.
In the conventional skew feeding correction portion, when a force of pressing the sheet toward the reference surface (force of moving the sheet toward the reference surface by the pair of skew-feed rollers) is too strong when the side end of the sheet abuts on the reference surface of the abutment reference member, the sheet is deflected, which might entail a jamming of the sheet or deterioration in correction precision.
There has been proposed a sheet conveying apparatus that changes a nip pressure of the skew-feed rollers to adjust the force of pressing the side end of the sheet toward the reference surface, in order to allow the side end of the sheet to be along the reference surface without the generation of the deflection, whereby the skew feeding is corrected (see U.S. Pat. No. 5,253,862).
In the sheet conveying apparatus described in U.S. Pat. No. 5,253,862, the nip pressure of the pair of skew-feed rollers is adjusted according to a correction method on a side registration basis, whereby the force of pressing the sheet toward the abutment reference member can be adjusted. However, the skew feeding correction is sometimes not sufficient. For example, problems described below might arise for a sheet having low stiffness property (hereinafter referred to as “stiffness”) such as a super thin sheet (a coated paper having a basis weight of less than 80 gsm), or for a case under high-temperature high-humidity environment in which a stiffness of a sheet is reduced owing to humidity.
Plural pairs of skew-feed rollers are arranged in the sheet conveying direction. When the pair of skew-feed rollers at the upstream side, which conveys a sheet as skewing at the beginning, nips the sheet, the sheet turns and abuts on the reference surface of the abutment reference member, whereby a deflection is generated on the side end of the sheet. In a sheet having high stiffness, the deflection is eliminated by the stiffness of the sheet. However, in a sheet having low stiffness, the deflection is not eliminated, and with this state, the sheet is nipped by the next pair of skew-feed rollers. Therefore, the skew feeding correction is not sufficiently performed owing to the difference in the magnitude of the deflection (deflection difference) caused in the sheet conveying direction. In particular, when the sheet S is conveyed as being nipped by plural pairs of skew-feed rollers as illustrated in FIG. 18B, the deflection is difficult to be eliminated. When the sheet is conveyed to the third pair of skew-feed rollers, the deflection might be buckled (folded) to cause a paper jam.
In view of this, the present invention aims to provide a sheet conveying apparatus provided with a skew feeding correction portion that can stably correct a skew feeding of a sheet, regardless of a type of sheet, and an image forming apparatus provided with the sheet conveying apparatus.